Creating ion energy distribution functions (IEDF)

ABSTRACT

Systems and methods for creating arbitrarily-shaped ion energy distribution functions using shaped-pulse-bias. In an embodiment, a method includes applying a positive jump voltage to an electrode of a process chamber to neutralize a wafer surface, applying a negative jump voltage to the electrode to set a wafer voltage, and modulating the amplitude of the wafer voltage to produce a predetermined number of pulses to determine an ion energy distribution function. In another embodiment a method includes applying a positive jump voltage to an electrode of a process chamber to neutralize a wafer surface, applying a negative jump voltage to the electrode to set a wafer voltage, and applying a ramp voltage to the electrode that overcompensates for ion current on the wafer or applying a ramp voltage to the electrode that undercompensates for ion current on the wafer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/433,204, filed Dec. 12, 2016, which isincorporated herein in its entirety.

FIELD

Embodiments of the present disclosure generally relate to systems andmethods for processing a substrate and, particularly, to systems andmethods for plasma processing of substrates.

BACKGROUND

A typical Reactive Ion Etch (RIE) plasma processing chamber includes aradiofrequency (RF) bias generator, which supplies an RF voltage to a“power electrode”, a metal baseplate embedded into the “electrostaticchuck” (ESC), more commonly referred to as the “cathode”. FIG. 1(a)depicts a plot of a typical RF voltage to be supplied to a powerelectrode in a typical processing chamber. The power electrode iscapacitively coupled to the plasma of a processing system through alayer of ceramic, which is a part of the ESC assembly. Non-linear,diode-like nature of the plasma sheath results in rectification of theapplied RF field, such that a direct-current (DC) voltage drop, or“self-bias”, appears between the cathode and the plasma. This voltagedrop determines the average energy of the plasma ions acceleratedtowards the cathode, and thus the etch anisotropy.

More specifically, ion directionality, the feature profile, andselectivity to the mask and the stop-layer are controlled by the IonEnergy Distribution Function (IEDF). In plasmas with RF bias, the IEDFtypically has two peaks, at low and high energy, and some ion populationin between. The presence of the ion population in between the two peaksof the IEDF is reflective of the fact that the voltage drop between thecathode and the plasma oscillates at the bias frequency. When a lowerfrequency, for example 2 MHz, RF bias generator is used to get higherself-bias voltages, the difference in energy between these two peaks canbe significant and the etch due to the ions at low energy peak is moreisotropic, potentially leading to bowing of the feature walls. Comparedto the high-energy ions, the low-energy ions are less effective atreaching the corners at the bottom of the feature (due to chargingeffect, for example), but cause less sputtering of the mask material.This is important in high aspect ratio etch applications, such ashard-mask opening.

As feature sizes continue to diminish and the aspect ratio increases,while feature profile control requirements get more stringent, itbecomes more desirable to have a well-controlled IEDF at the substratesurface during processing. A single-peak IEDF can be used to constructany IEDF, including a two-peak IEDF with independently controlled peakheights and energies, which is very beneficial for high-precision plasmaprocessing. Creating a single-peak IEDF requires having anearly-constant voltage at the substrate surface with respect to plasma,i.e. the sheath voltage, which determines the ion energy. Assumingtime-constant plasma potential (which is typically close to zero or aground potential in processing plasmas), this requires maintaining anearly constant voltage at the substrate with respect to ground, i.e.substrate voltage. This cannot be accomplished by simply applying a DCvoltage to the power electrode, because of the ion current constantlycharging the substrate surface. As a result, all of the applied DCvoltage would drop across the substrate and the ceramic portion of theESC (i.e., chuck capacitance) instead of the plasma sheath (i.e., sheathcapacitance). To overcome this, a special shaped-pulse bias scheme hasbeen developed that results in the applied voltage being divided betweenthe chuck and the sheath capacitances (we neglect the voltage dropacross the substrate, as its capacitance is usually much larger than thesheath capacitance). This scheme provides compensation for the ioncurrent, allowing for the sheath voltage and the substrate voltage toremain constant for up to 90% of each bias voltage cycle. Moreaccurately, this biasing scheme allows maintaining a specific substratevoltage waveform, which can be described as a periodic series of shortpositive pulses on top of the negative dc-offset (FIG. 1(b)). Duringeach pulse, the substrate potential reaches the plasma potential and thesheath briefly collapses, but for ˜90% of each cycle the sheath voltageremains constant and equal to the negative voltage jump at the end ofeach pulse, which thus determines the mean ion energy. FIG. 1(a) depictsa plot of a special shaped-pulse bias voltage waveform developed tocreate this specific substrate voltage waveform, and thus enable keepingthe sheath voltage nearly constant. As depicted in FIG. 2, theshaped-pulse bias waveform includes: (1) a positive jump to remove theextra charge accumulated on the chuck capacitance during thecompensation phase; (2) a negative jump (V_(OUT)) to set the value ofthe sheath voltage (V_(SH))—namely, V_(OUT) gets divided between thechuck and sheath capacitors connected in series, and thus determines(but is generally larger than) the negative jump in the substratevoltage waveform; and (3) a negative voltage ramp to compensate for ioncurrent and keep the sheath voltage constant during this long “ioncurrent compensation phase”. We emphasize that there can be othershaped-pulse bias waveforms that also allow maintaining a specificsubstrate voltage waveform shown in FIG. 1(b) (characterized by thenearly constant sheath voltage), and are hence capable of producing amono-energetic IEDF. For example, if the electrostatic chuck capacitanceis much larger than the sheath capacitance, the negative voltage rampphase described in (3) above can be substituted with a constant voltagephase. Some of the systems and methods proposed below can also beimplemented with these other shaped-pulse bias waveforms, and we will bemaking a special note of that wherever applicable.

While a single-peak IEDF is widely considered to be a highly desirableshape of IEDF resulting in improved selectivity and feature profile, insome etch applications an IEDF having a different shape, such as a widershaped IEDF, is required.

SUMMARY

Systems and methods for creating arily-shaped ion energy distributionfunctions using shaped-pulse bias are provided herein.

In some embodiments, a method includes applying a shaped pulse bias toan electrode of a process chamber and modulating the amplitude of thenegative voltage jump (V_(OUT)), and hence the sheath voltage (V_(SH)),in a predetermined manner, such that the relative number of pulses at aspecific amplitude determines the relative ion fraction at the ionenergy, corresponding to this amplitude We emphasize that this schemecan be implemented with any shaped-pulsed bias waveforms (notnecessarily the one shown in FIG. 1(a) that allow maintaining a specificsubstrate voltage waveform shown in FIG. 1(b) (characterized by thenearly constant sheath voltage), and are hence capable of producing amono-energetic IEDF.

In some other embodiments, a method includes applying a shaped pulsebias with the voltage waveform shown in FIG. 1(a), and creating avoltage ramp during ion compensation phase that has a more negativeslope (dV/dt) than is required to maintain a constant substrate voltage,i.e. overcompensating for the ion current. In some other embodiments, amethod includes applying a shaped pulse bias with the voltage waveformshown in FIG. 1(a), and creating a voltage ramp during ion compensationphase that has a less negative slope (dV/dt) than is required tomaintain a constant substrate voltage, i.e. undercompensating for theion current.

Other and further embodiments of the present disclosure are describedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure, briefly summarized above anddiscussed in greater detail below, can be understood by reference to theillustrative embodiments of the disclosure depicted in the appendeddrawings. However, the appended drawings illustrate only typicalembodiments of the disclosure and are therefore not to be consideredlimiting of scope, for the disclosure may admit to other equallyeffective embodiments.

FIG. 1(a) depicts a plot of a special shaped-pulse developed to enablekeeping the sheath voltage constant.

FIG. 1(b) depicts a plot of a specific substrate voltage waveformresulting from the biasing scheme of FIG. 1(a) which allows the sheathvoltage and the substrate voltage to remain constant for up to 90% ofeach bias voltage cycle.

FIG. 1(c) depicts a plot of a single-peak IEDF resulting from thebiasing scheme of FIG. 1(a).

FIG. 2 depicts a substrate processing system in which embodiments inaccordance with the present principles can be applied.

FIG. 3 depicts a plot of voltage pulses to set a value of substratevoltage in accordance with an embodiment of the present principles.

FIG. 4 depicts a graphical representation of a resulting IEDF for theselected voltage pulses of FIG. 3 in accordance with an embodiment ofthe present principles

FIG. 5 depicts a plot of the special shaped-pulse of FIG. 1 modified toovercompensate and undercompensate for ion current in accordance withembodiments of the present principles.

FIG. 6, depicts a plot of induced voltage pulses on the wafer resultingfrom the special shaped-pulse bias of FIG. 5.

FIG. 7 depicts a graphical representation of a resulting IEDF for thevoltage pulses of FIG. 6 in accordance with an embodiment of the presentprinciples.

FIG. 8 depicts a flow diagram of a method for the creation of anarbitrarily-shaped ion energy distribution function in accordance withan embodiment of the present principles.

FIG. 9 depicts a flow diagram of a method for the creation of anarbitrarily-shaped ion energy distribution function in accordance withanother embodiment of the present principles.

FIG. 10 depicts a flow diagram of a method for the creation of anarbitrarily-shaped ion energy distribution function in accordance withanother embodiment of the present principles.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The figures are not drawn to scale and may be simplifiedfor clarity. Elements and features of one embodiment may be beneficiallyincorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

Systems and methods for creating arbitrarily-shaped ion energydistribution functions using shaped-pulse-bias are provided herein. Theinventive systems and methods advantageously facilitate the creation ofarbitrarily-shaped ion energy distribution function (IEDF) by modulatingan amplitude of a shaped-pulse bias waveform. Embodiments of theinventive methods can advantageously provide shaping of the voltagewaveform to provide arbitrary IEDF shapes, for example, an IEDF with awider profile. In the description herein the terms wafer and substrateare used interchangeably.

FIG. 2 depicts a high level schematic diagram of a substrate processingsystem 200 in which embodiments in accordance with the presentprinciples can be applied. The substrate processing system 200 of FIG. 2illustratively includes a substrate support assembly 205, and a biassupply 230. In the embodiment of FIG. 2, the substrate support assembly205 includes a substrate support pedestal 210, a power electrode 213 anda layer of ceramic 214 separating the power electrode 213 from a surface207 of the substrate support assembly 205. In various embodiments, thesystem 200 of FIG. 2 can comprise components of a plasma processingchamber such as the SYM3®, DPS®, ENABLER®, ADVANTEDGE™ and AVATAR™process chambers available from Applied Materials, Inc. of Santa Clara,Calif. or other process chambers.

In some embodiments, the bias supply 230 includes a memory for storingcontrol programs and a processor for executing the control programs tocontrol the voltage to be provided by the bias supply 230 to the powerelectrode 213 and at least modulate an amplitude of a wafer voltage toproduce a predetermined number of pulses and, alternatively or inaddition, apply a negative jump voltage to the electrode to set a wafervoltage for the wafer or apply a ramp voltage to the electrode thatovercompensates or undercompensates for ion current on the wafer inaccordance with embodiments of the present principles described herein.In alternate embodiments, the substrate processing system 200 of FIG. 2can include an optional controller 220 including a memory for storingcontrol programs and a processor for executing the control programs forcommunicating with the bias supply 230 for at least controlling thevoltage to be provided by the bias supply 230 to the power electrode 213and to at least modulate an amplitude of a wafer voltage to produce apredetermined number of pulses and, alternatively or in addition, applya negative jump voltage to the electrode to set a wafer voltage for thewafer or apply a ramp voltage to the electrode that overcompensates orundercompensates for ion current on the wafer in accordance withembodiments of the present principles described herein.

In operation, a substrate to be processed is positioned on a surface ofthe substrate support pedestal 210. In the system 200 of FIG. 2, avoltage (shaped pulse bias) from the bias supply 230 is supplied to thepower electrode 213. Non-linear, diode-like nature of the plasma sheathresults in rectification of the applied RF field, such that adirect-current (DC) voltage drop, or “self-bias”, appears between thecathode and the plasma. This voltage drop determines the average energyof the plasma ions accelerated towards the cathode. Ion directionalityand the feature profile are controlled by the Ion Energy DistributionFunction (IEDF). The bias supply 230 can supply a special shaped pulsebias to the power electrode 213 in accordance with embodiments of thepresent principles described herein. This biasing scheme allowsmaintaining a specific substrate voltage waveform, which can bedescribed as a periodic series of short positive pulses on top of thenegative dc-offset (FIG. 1(b)). During each pulse, the substratepotential reaches the plasma potential and the sheath briefly collapses,but for ˜90% of each cycle the sheath voltage remains constant and equalto the negative voltage jump at the end of each pulse, which thusdetermines the mean ion energy.

Referring back to FIG. 1(a), the amplitude of the shaped-pulse biassignal, and hence the wafer voltage is represented by V_(out). Theinventors determined that, in at least some embodiments in accordancewith the present principles, the shape of the IEDF can be controlled bymodulating the amplitude and frequency of the shaped-pulse bias signal.This method includes applying a shaped pulse bias to an electrode of aprocess chamber and modulating the amplitude of the negative voltagejump (V_(OUT)), and hence the sheath voltage (V_(SH)), in apredetermined manner, such that the relative number of pulses at aspecific amplitude determines the relative ion fraction at the ionenergy, corresponding to this amplitude. The number of pulses at eachamplitude must be sufficient to account for transition from one sheathvoltage to the next, during which the respective ESC charge isestablished. The burst comprising the trains of pulses with givenamplitudes (FIG. 3) is then repeated over and over for the duration ofthe process step. Active bursts (on-phases) can be interleaved withperiods of silence (off-phases). The duration of each on-phase relativeto the total duration of the burst (on and off phases combined) isdetermined by the duty cycle, and the total duration of the burst(period) is equal to the inverse of the burst frequency. Alternatively,each burst may be composed of a series of pulses with a given (and thesame) amplitude, and the train of bursts with different amplitudes isthen used to define an IEDF. The relative number of bursts (in a train)with a given amplitude determines the relative portion of ions at aspecific energy, and the negative jump amplitude (V_(OUT)) of the pulsesin these bursts determines the ion energy. The predefined train ofbursts is then repeated over and over for the duration of the recipestep. For example, to create a two-peak IEDF with 25% ions contained inthe low-energy peak, and 75% of ions contained in the high-energy peak,the train of bursts needs to be composed of 3 bursts of pulses with thenegative jump amplitude corresponding to the high ion energy and 1 burstof pulses with the amplitude corresponding to the low ion energy. Suchtrain may be designated as “HHHL”. In turn, to create an IEDF with 3energy peaks of equal height—high (H), mid (M), and low (L)—the train of3 bursts with different amplitudes corresponding to H, M and L ionenergies is required, and may be designated as “HML”. A single-peak IEDFis produced by a train composed of a single burst (with both on and offphases) of pulses with a predefined negative jump amplitude. Weemphasize that this scheme can be implemented with any shaped-pulsedbias waveforms (not necessarily the one shown in FIG. 1(a)) that allowmaintaining a specific substrate voltage waveform shown in FIG. 1(b)(characterized by the nearly constant sheath voltage), and are hencecapable of producing a mono-energetic IEDF.

For example, FIG. 3 depicts a plot of voltage pulses to be supplied by apower supply to an electrode of a processing chamber to set a value ofsubstrate voltage in accordance with an embodiment of the presentprinciples. In the embodiment of FIG. 3, the full jump of the wafervoltage determines the ion energy, whereas the number of pulses (e.g.,the total time duration) corresponding to the voltage jump determinesthe relative ion fraction at this energy (i.e., the IEDF).

FIG. 4 depicts a graphical representation of a resulting IEDF for theselected voltage pulses of FIG. 3 in accordance with an embodiment ofthe present principles. As depicted in FIG. 4, the multiple voltagepulses of FIG. 3 result in a wider IEDF, which can be advantageous insuch applications as hard-mask open high-aspect ratio etch, whichrequire wider ion energy distribution.

The control of the amplitude and frequency of the voltage pulsessupplied by a power supply to an electrode of a processing chamber inaccordance with the present principles is able to provide awell-controlled and well-defined IEDF shape required by a particularetch process and application.

In another embodiment in accordance with the present principles, amethod includes applying a shaped pulse bias with the voltage waveformshown in FIG. 1(a), and creating a voltage ramp during ion compensationphase that has a more negative slope (dV/dt) than is required tomaintain a constant substrate voltage, i.e. overcompensating for the ioncurrent. This results in a substrate voltage waveform shown in FIG. 6where the magnitude of the substrate voltage (and hence sheath voltageand instantaneous ion energy) increases during the ion currentcompensation phase. This creates ion energy spread and anon-monoenergetic IEDF shown in FIG. 7 with IEDF width controlled by thenegative slope of the applied shape-pulse bias waveform. For example,FIG. 5 depicts a plot of the special shaped-pulse of FIG. 1(a) modifiedto overcompensate for ion current that charges the wafer in accordancewith an embodiment of the present principles. As depicted in FIG. 5, thevoltage ramp of FIG. 1(a) intended to compensate for ion current thatcharges the wafer is modified in the special shaped-pulse of FIG. 5 ofthe present principles to overcompensate for the ion current thatcharges the wafer. As depicted in FIG. 5, the positive jump of FIG. 1intended to neutralize the wafer surface, no longer neutralizes thewafer surface in the special shaped-pulse of FIG. 5 of the presentprinciples.

FIG. 6, depicts a plot of induced voltage pulses on the wafer resultingfrom the special shaped-pulse of FIG. 5. As depicted in FIG. 6, thevoltage jump determines the ion energy and the energy width isdetermined by minimum and maximum wafer voltage jumps during the cycle.

FIG. 7 depicts a graphical representation of a resulting IEDF for thevoltage pulses of FIG. 6 in accordance with an embodiment of the presentprinciples. As depicted in FIG. 7, the IEDF resulting from theapplication of the overcompensated special shaped-pulse of FIG. 5includes a wider, double-peaked profile in which V_(min) and V_(max)determine the IEDF width however do not necessarily coincide with theenergy peaks. The overcompensation in accordance with the presentprinciples enables a higher precision of control than can be achieved bymixing 2 RF frequencies (e.g. 2 and 13.56 MHz).

In another embodiment in accordance with the present principles, amethod includes applying a shaped pulse bias with the voltage waveformshown in FIG. 1(a), and creating a voltage ramp during ion compensationphase that has a less negative slope (dV/dt) than is required tomaintain a constant substrate voltage, i.e. undercompensating for theion current. This results in a substrate voltage waveform shown in FIG.6 where the magnitude of the substrate voltage (and hence sheath voltageand instantaneous ion energy) decreases during the ion currentcompensation phase. This creates ion energy spread and anon-monoenergetic IEDF shown in FIG. 7 with IEDF width controlled by thenegative slope of the applied shape-pulse bias waveform. For example,referring back to FIG. 5, FIG. 5 depicts a plot of the specialshaped-pulse of FIG. 1 modified to undercompensate for ion current thatcharges the wafer in accordance with an embodiment of the presentprinciples. As depicted in FIG. 5, the voltage ramp of FIG. 1 intendedto compensate for ion current that charges the wafer is modified in thespecial shaped-pulse of FIG. 5 of the present principles toundercompensate for the ion current that charges the wafer. As depictedin FIG. 5, the positive jump of FIG. 1 intended to neutralize the wafersurface, no longer neutralizes the wafer surface in the specialshaped-pulse of FIG. 5 of the present principles.

Referring back to FIG. 7, a graphical representation of a resulting IEDFfor the undercompensating of an embodiment of the present principles isdepicted. As depicted in FIG. 7, the IEDF resulting from the applicationof the undercompensated special shaped-pulse of FIG. 5 includes a wider,single-peaked profile.

FIG. 8 depicts a flow diagram of a method for the creation of anarbitrarily-shaped ion energy distribution function in accordance withan embodiment of the present principles. The method 800 can begin at 802during which a negative jump voltage is applied to the electrode to seta wafer voltage. The method 800 can then proceed to 804.

At 804, the amplitude of the wafer voltage is modulated to produce apredetermined number of pulses to determine an ion energy distributionfunction.

The method 800 can then be exited.

FIG. 9 depicts a flow diagram of a method for the creation of anarbitrarily-shaped ion energy distribution function in accordance withanother embodiment of the present principles. The method 900 can beginat 902 during which a positive jump voltage is applied to an electrodeof a process chamber to neutralize a wafer surface. The method 900 canthen proceed to 904.

At 904, a negative jump voltage is applied to the electrode to set awafer voltage. The method 900 can then proceed to 906.

At 906, a ramp voltage is applied to the electrode that overcompensatesfor ion current on the wafer. The method 900 can then be exited.

FIG. 10 depicts a flow diagram of a method for the creation of anarbitrarily-shaped ion energy distribution function in accordance withanother embodiment of the present principles. The method 1000 can beginat 1002 during which a positive jump voltage is applied to an electrodeof a process chamber to neutralize a wafer surface. The method 1000 canthen proceed to 1004.

At 1004, a negative jump voltage is applied to the electrode to set awafer voltage. The method 1000 can then proceed to 1006.

At 1006, a ramp voltage is applied to the electrode thatundercompensates for ion current on the wafer. The method 1000 can thenbe exited.

While the foregoing is directed to embodiments of the presentdisclosure, other and further embodiments of the disclosure may bedevised without departing from the basic scope thereof.

The invention claimed is:
 1. A method, comprising: applying a negativejump voltage to an electrode of a process chamber to set a wafer voltagefor a wafer; and modulating the wafer voltage at different amplitudes tocreate an ion energy distribution function having more than one energypeak with different amplitudes, wherein a relative number of pulses at aspecific amplitude determines a relative ion fraction at an ion energycorresponding to the specific amplitude.
 2. The method of claim 1,comprising: applying a positive jump voltage to the electrode of theprocess chamber to neutralize a surface of the wafer.
 3. The method ofclaim 2, wherein the positive jump voltage is applied to the electrodeof the process chamber prior to applying the negative jump voltage. 4.The method of claim 1, wherein the modulation of the wafer voltage atdifferent amplitudes controls a feature profile of a resulting ionenergy distribution function.
 5. The method of claim 1, wherein thewafer voltage is modulated at different amplitudes to create a desiredion energy distribution function.
 6. The method of claim 5, wherein thedesired ion energy distribution function is created to induce a specificbias voltage waveform on the wafer.
 7. The method of claim 1,comprising: modulating the wafer voltage at different points in time tocreate an ion energy distribution function having more than one energypeak.
 8. The method of claim 7, wherein an ion fraction for each of theenergy peaks is determined by a number of pulses produced during arespective modulation of the wafer voltage at the different points intime.
 9. The method of claim 1, wherein an ion fraction for each of theenergy peaks is determined by a number of pulses produced during arespective modulation of the wafer voltage at the different amplitudes.10. The method of claim 1, comprising: applying a positive jump voltageto the electrode of the process chamber prior to applying the negativejump voltage to the electrode to neutralize a surface of a wafer; andapplying a ramp voltage to the electrode that overcompensates for ioncurrent on the wafer.
 11. The method of claim 10, wherein applying aramp voltage to the electrode that overcompensates for ion current onthe wafer comprises applying a ramp voltage to the electrode thatcomprises a slope that is more negative than is required to maintain aconstant voltage on the wafer.
 12. The method of claim 10, wherein aminimum voltage and a maximum voltage of a current induced on the waferdetermine a width of a resulting ion energy distribution function. 13.The method of claim 10, comprising: adjusting a slope of the rampvoltage to create a desired ion energy distribution function.
 14. Themethod of claim 13, wherein the desired ion energy distribution functionis created to induce a specific bias voltage waveform on the wafer. 15.The method of claim 1, comprising: applying a positive jump voltage tothe electrode of the process chamber prior to applying the negative jumpvoltage to the electrode to neutralize a surface of a wafer; andapplying a ramp voltage to the electrode that undercompensates for ioncurrent on the wafer.
 16. The method of claim 15, wherein applying aramp voltage to the electrode that undercompensates for ion current onthe wafer comprises applying a ramp voltage to the electrode thatcomprises a slope that is less negative than is required to maintain aconstant voltage on the wafer.
 17. The method of claim 15, wherein aminimum voltage and a maximum voltage of a current induced on the waferdetermine a width of a resulting ion energy distribution function. 18.The method of claim 15, comprising: adjusting a slope of the rampvoltage to create a desired ion energy distribution function.
 19. Themethod of claim 18, wherein the desired ion energy distribution functionis created to induce a specific bias voltage waveform on the wafer.